The reaction between CH and butadiene (CH\(_2\) CHCHCH\(_2\) ) is calculated to produce
\(c\) -C\(_5\) H\(_6\)  with no barrier [1]}. Adopting a rate coefficient of
10\(^{-10}\)  cm\(^3\)  s\(^{-1}\)  for this reaction in our chemical model, based on [2]},
and assuming that c-C5H6 is mostly removed through reactions with C atoms and C\(^+\) 
and H\(^+\)  ions, we calculate a peak abundance of \(\sim \)  3 \(\times \)  10\(^{-13}\) 
relative to H\(_2\)  for \(c\) -C\(_5\) H\(_6\) . This is more than three orders of magnitude
below the observed value.
In the chemical model,
butadiene is essentially formed by the reaction CH + CH\(_2\) CHCH\(_3\) 
[3]}, [4]}, [5]} with a peak abundance of \(\sim \)  3 \(\times \) 10\(^{-10}\) 
relative to H\(_2\) . Butadiene is an interesting potential precursor of \(c\) -C\(_5\) H\(_6\) , and also
of benzene [6]}, but to play such a role it needs to have an abundance much higher
than calculated by current gas-phase chemical models. Unfortunately, it is a non-polar molecule
and thus it cannot be detected through radio techniques.
